The invention relates to an optical scanning device for apparatuses for recording and/or reproducing information on optical recording media, which is suitable in particular for recording and/or reproducing information at high speed, such as, for example, the scanning of a DVD at a twelve-fold speed.
Optical scanning devices for recording media in laser disc and compact disc players are generally known. The design and function of such scanning devices, also referred to as pick-up, are described in Electronic Components and Applications, Vol. 6, No. 4, 1984, pages 209-215. Such scanning devices have a so-called actuator on which there is arranged an objective lens which is provided for tracking and for focusing the light beam or laser beam on the optical recording medium. In principle, optical scanning devices can be distinguished by the way in which the objective lens is suspended. Thus, the objective lens mount is fastened to a frame for example by means of four parallel leaf springs in the case of a known leaf-spring actuator or via articulated joints or hinges in the case of an actuator of the articulated-joint or hinge type, respectively. Disadvantages are that such arrangements have an undesirable tendency to oscillate, require a high level of assembly complexity and have relatively high reaction times inter alia due to the mass to be moved. A further way of elastically supporting the objective lens mount is to use four wires which connect the lens holder to the actuator baseplate. In comparison with optical scanning devices with a leaf spring or articulated joint, so-called wire pick-ups can be produced more cost-effectively and allow shorter access times. However, they are particularly sensitive relative to other parallel guides with regard to resonance phenomena, tilting of the objective lens and thermal loads. On the other hand, in particular for high-speed applications and in order to achieve short access times, high currents are necessary in the track and focus coils, which currents heat the lens holder and lead to undesirable oscillation of the lens holder and to tilting of the objective lens. The scanning of optical recording media at a multiple of the speed which is necessary for reading or recording on an audio, video or data CD or DVD at single speed increases the requirements made of optical scanning devices with regard to uniformity of the response characteristic. The frequency response and phase angle of the optical scanning device should have as uniform a profile as possible over a wide frequency range and not be disturbed by free oscillations. It is demanded, for example, that resonance phenomena occur only above 40 kilohertz and they have a level reduction or attenuation of 45 decibels in comparison with attenuation at a frequency of 1 kilohertz, in order to be able to scan a DVD at a twelve-fold speed.
The object of the invention is to provide an optical scanning device for optical recording media which to the greatest possible extent avoids the disadvantages of known optical scanning devices with regard to oscillation properties, speed and thermal load ability, meets the requirements in the course of reading or recording at high speed and can be produced with a low degree of complexity.
This object is achieved by means of features which are specified in the main claims. Advantageous designs and developments are specified in subclaims.
One aspect of the invention involves avoiding to the greatest possible extent the disadvantages of so-called wire pick-ups relative to other parallel guides with regard to tilting of the objective lens, asymmetrical guidance properties, natural resonance behaviour and thermal load ability for high-speed applications, and nevertheless producing the optical scanning device with a low degree of complexity.
To that end, an optical scanning device for high-speed applications is provided which has a lens holder which is formed by a hollow body and whose side walls are arranged such that they run at an angle to a connecting point with its elastic support. The lens holder has a hexagonal form which is formed with the side walls which run at an angle to a connecting point with its elastic support. As a result of the hexagonal shape of the lens holder, air chambers are formed between lens and coils of the actuator, the distance between lens and coils of the actuator is enlarged and the stability or rigidity of the lens holder formed by a hollow body is increased. For this purpose, the side walls or side areas of the lens holder are preferably arranged such that they deviate at an angle of from 3 to 10 degrees from a straight line. The angle at which the side walls of the lens holder are arranged with respect to one another is chosen in such a way that, in the event of maximum deflection of the lens holder from its neutral position, contact occurs neither before nor after the connecting point with the wires which support it.
The lens holder formed by a hollow body is of integral design and preferably constructed symmetrically. The configuration as hollow body with air chambers further improves the thermal properties of the optical scanning device since additional cooling and heat dissipation are achieved by actuator movements. Moreover, the heat dissipation can be increased further by giving the surface of the lens holder a dark configuration. Furthermore, the wires which elastically support the lens holder are arranged at an angle which, in the neutral position of the lens holder, deviates by more than two degrees from a perpendicular to the lens holder, in order to ensure little or no tilting of the lens over the entire deflection range of the actuator for tracking. With the angle, a change is made to the rigidity of the support of the actuator on its baseplate and, as a result, also to the resonance behaviour in a targeted manner for a direction of action. By virtue of the angle at which the wires which elastically support the lens holder for tracking are brought up to the lens holder, there is an advantageous influence both on the guidance properties in the event of deflection of the lens for the purpose of following the recording track, on account of the lens tilting to a lesser extent, and on the natural resonance behaviour of the optical scanning device. Since the ends of the wires, which ends project freely beyond the connecting point with the lens holder, may, as a result of the mounting process or during separation, deviate from the original direction in which they were brought up to the lens holder, the side walls of the lens holder are arranged at an angle which is preferably greater than or equal to the angle at which the wires are brought up to the lens -holder in its neutral position. The wires which elastically support the lens holder for tracking on the baseplate are brought up to the lens holder in punctiform fashion. This enlarges the lateral distance between wire and lens holder in the vicinity of the soldering or connection point, with the result that, on the one hand, the process of bringing up the wires is facilitated and, on the other hand, an adverse effect on the guidance properties which is based on burr on the parts or on residues of soldering agent is avoided. Furthermore, it may be assumed that reduced resonance phenomena deep into the kilohertz range can also be attributed to the fact that oscillations proceeding from the lens holder are prevented from propagating by the angle between wire and side wall of the lens holder.
The side walls which are arranged at an angle to one another have the effect of achieving, despite a compact structure of the lens holder, a larger distance between the lens and the windings, which are heated by high currents in high-speed applications.
A depression for accommodating the focus coil is provided in the side walls of the lens holder, the said depression forming webs on which there is fixed, after the application of the focus coil, a circuit board, preferably formed by a printed circuit board. The circuit board advantageously connects to one another the webs which are provided on the upper and lower edges of the lens holder, and thereby contributes to the stability of the lens holder. Such a circuit board is arranged on each side of the lens holder and soldering points are provided on the circuit board, not only ends of the coils but also the wires which are provided for elastic support of the lens holder being fixed to the said soldering points. The circuit boards are arranged on the side areas, which run at an angle to one another, in such a way that an edge of the circuit board coincides approximately with the line formed by side areas which meet one another at an angle. The fixing points provided for the connection of the wires which elastically support the lens holder are also arranged in this region, thereby ensuring a punctiform connection of these wires. In order to align the circuit board on the lens holder, pins are provided on the webs of the lens holder and engage in the recesses in the circuit board, and either a latching connection or an adhesive bond is provided in order to connect a circuit board and lens holder. Further advantages of the circuit boards connecting the webs of the lens holder are that they have a low weight and can be of symmetrical design. Furthermore, pins are also available in the case of a symmetrically constructed lens holder. The said pins can be used for attaching winding ends. The focus coil is preferably arranged symmetrically with respect to the horizontal centre of mass of the lens holder and the lens holder webs bounding the winding space of the focus coil preferably have shoulders at the ends of the inner faces of the webs. The effect achieved by the shoulders which are provided in one embodiment of the lens holder is that the turns of the focus coil are wound adjacently without overlapping during the winding operation. This is particularly important for the attachment of the first winding, since turns otherwise cross one another in an undesirable manner. In accordance with a first design, the lens holder is produced using plastic injection-moulding technology and additional ventilation openings are provided in the region surrounding the lens, which openings reduce the air resistance of the actuator and intensify cooling during its movement. The assembly comprising lens holder, focus and tracking coils and the abovementioned circuit board and lens is designated as actuator. However, in the process for producing the optical scanning device, the lens is generally inserted into the lens holder only after the lens holder has been mounted on an actuator baseplate. The mounting process for the lens is simplified by guide webs provided on the lens holder. Air chambers formed by the hexagonal form of the lens holder separate the lens from the winding chamber by a cavity which reduces the thermal load on the lens on account of high coil currents in high-speed applications. The hexagonal form of the thin-walled lens holder increases the rigidity of the lens holder by comparison with a rectangular form and the winding of the focus coil is improved by the wire tension that occurs, since the coil wire bears better on the side walls on the hexagonal lens holder in comparison with a rectangular lens holder. The hexagonal form of the lens holder and a coil which does not project beyond the lens holder reduce the air resistance during lateral movement.
Peripherally arranged webs of the lens holder which is produced from plastic are advantageously used for aligning the tracking coils, which makes it possible to dispense with a complicated positioning device. The tracking coils partially widen the webs and stiffen them. The natural resonance behaviour is thereby improved further. In order to improve the heat dissipation, the lens holder is coloured or coated such that it is dark, preferably black, as a result of which higher coil currents can be permitted and the actuator can be operated at high frequencies with a large power, which is of particular importance for example for focusing the light beam or laser beam on the information medium for recording or reproducing information. In accordance with a further design, the pins which are not required for positioning the circuit boards in the case of a symmetrical lens holder are used for the purpose of having winding wire wrapped around them or for the purpose of fixing coil wire ends. In accordance with a further design, the lens holder is provided with an increased wall thickness in the connection region of the wires which form the elastic support for the lens holder, the said increased wall thickness absorbing forces proceeding from the circuit boards, without deforming the lens holder. The rigidity and the natural resonant frequency of the lens holder are thereby increased further. Furthermore, in order to increase the strength of the side walls, a rib running at an inclination may be arranged between the ventilation openings surrounding the lens. In accordance with a further design, a focus coil is used which is formed by copper-sheathed aluminium wire. Aluminium-core wire is lighter than comparable solid-copper wire, so that forces generated in the focus direction are increased by a higher number of turns given the same weight. The same applies to the tracking coils. For tracking, coils which are wound in a D-shaped manner are preferably used. The outer part of the coil not contributing to force generation is reduced in length and the efficiency is thus improved. The leakage fields which are generated by the outer part of the coil in a rectangular coil and counteract the force generated in the inner region and thus lead to losses are reduced. The side walls of the lens holder are connected via transverse walls which preferably have a cutout in order to prevent rotation of the lens holder in the mounting device. The transverse walls contribute to stiffening the lens holder and a reduction in the weight of the lens holder is achieved by cutouts in the said walls.
The depth of the cutouts is chosen in such a way that the centre of gravity of the lens holder lies exactly between the wires which elastically support the lens holder. The abovementioned design measures can be used individually or in combination for the production of an optical scanning device for high-speed applications. In a further embodiment of the optical scanning device for high-speed applications, the lens holder is designed as a cast metal part. Metal materials and plastics materials have fundamentally different properties with regard to thermal conductivity and mechanical stability, which have to be taken into account in the configuration of an optical scanning device for high-speed applications. The metallic lens holder, for which the metal material used is preferably magnesium, is likewise preferably coloured black, or at least has a dark surface, for the purpose of heat dissipation to the surroundings. Since webs are difficult to realise in metal casting technology, in this design webs have been dispensed with to the greatest possible extent and, moreover, no ventilation holes are provided in addition to the air chambers. Nevertheless, the natural resonance behaviour is sufficient on account of the high material rigidity of the metal material and sufficient heat dissipation is also provided on account of the thermal conductivity of the metal material, the said heat dissipation being achieved in particular by virtue of the configuration of the lens holder as hollow body with air chambers. As a result of the webs being dispensed with, the hexagonal form of the lens holder corresponds to an H with limbs arranged such that they run towards one another at an angle. Since there are no webs in the region of the openings of the H-form, the tracking coils are positioned in the known manner in this design. In terms of its fundamental structure, however, the lens holder is designed in a manner corresponding to the first design. The wires which elastically support the lens holder in particular for tracking on the baseplate are brought up to the lens holder such that they run at an angle to one another and the lens holder is configured in such a way that it has side walls which are provided such that they run at an angle proceeding from the connecting point of the lens holder with the wires. The lens holder which is produced as a cast metal part likewise has a depression for accommodating the focus coil in the side walls. Irrespective of the design as a metallic lens holder or a lens holder produced from plastics material, the lens holder is preferably constructed symmetrically and the connecting point of the lens holder with the wires is preferably provided at the centre of gravity or at least in the region of the centre of gravity. The wires of the wire pick-up which form an elastic support for the lens holder are preferably pretensioned during connection to the lens holder by copper-bit soldering, in order to achieve less tilting and better guidance properties. However, it is possible to omit the pretensioning of the wires if connecting methods that result in a lower level of loading are chosen for fixing the wires. Examples of such connecting methods are laser soldering or the use of conductive adhesives.
The circuit board which connects the webs of the lens holder on the side walls and is used for the connection of the wires which elastically support the lens holder preferably has further depressions or openings in which winding wires are guided to the soldering or connection points provided on the circuit board. As a result, the attachment of the winding wires to the circuit board is simplified and the winding wires are held in a defined position relative to the soldering point prior to soldering, thereby simplifying the soldering process.
By virtue of the compact structure of the lens holder, which is achieved by a hollow body which has air chambers and whose side walls are arranged such that they run at an angle to the connecting point with its elastic support, and with an elastic support which is brought up to the lens holder or actuator at an angle which deviates from 90 degrees, an optical scanning device has been created which has an undisturbed frequency response and a corresponding phase characteristic over a large frequency range extending deep into the kilohertz range, which constitute a prerequisite for the possibility of using the optical scanning device for recording or for reproducing information at high speed. Furthermore, the particular configuration of the optical scanning device has achieved an increased attenuation or a larger response margin of the resonance value relative to a value occurring at a frequency of 1 kilohertz.